Physiological sensing units of the electrochemical type are commonly used for non-invasive transcutaneous measurement of partial pressure of gases, e.g., oxygen and carbon dioxide, in blood and body tissue. Exemplary of such electrochemical sensors are those of the Clark type typically used for polarographic oxygen measurement and those of the Stow-Gertz-Severinghaus type typically used for ion-sensitive carbon dioxide measurement. In operation, a membrane is uniformly and tautly stretched over the measuring surface of the sensing unit, with a very thin film of hygroscopic electrolyte uniformly disposed between the measuring surface and the membrane. The membrane is positioned and retained in contact with the measuring surface by a removable retaining assembly such as the assembly described in commonly owned U.S. Pat. No. 4,425,918, which assembly has gained acceptance in the industry and has proved to be quite commercially successful.
A particular problem associated with such physiological sensors is, that during use, water vapor can diffuse through the outer surface of the diaphragm and into the electrolyte film which water vapor increases the film thickness resulting in undesirable effects, particularly, an increase in the diffusion resistance of the gas being measured as well as an increase in the conductance of the electrolyte. Consequently, the sensitivity of the sensor can be adversely affected and response time to changes in gas concentration can be unduly prolonged.
In order to avoid such change in film thickness of the electrolyte, the sensor element itself could be provided with compensating capillaries or channels which, for example, could be incorporated into the encircling mating-surface rim. On the one hand, however, this is not possible with respect to sensors already in the field. On the other hand, relatively thick capillaries would be required in order to maintain function when wear is caused by repeated covering and despite unavoidable depositing, for example, of electrolyte residues. Also, in state of the art measuring sensors, this could in turn promote premature migration of the electrolyte.